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Old 7th December 2012, 09:26 AM   #11
Bonsai is offline Bonsai  Taiwan
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wow - classic old amp! Fantastic
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Old 7th December 2012, 12:00 PM   #12
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Quote:
Originally Posted by MikeBettinger View Post
I remove the transistors and leave the rest. As for the replacing the output transistors, if it still has the original Toshiba 2SD424/2SB554s I use them as they are good and GAS matched them well. A quick check of the voltage drop across the .2 ohm emitter resistors to verify if the bias between the transistors is close (<5mv between all) this is a good gauge of whether or not to change them. The high speed transistors might need a different compensation cap on the voltage amp transistors if there is any tendency to oscillate. The current cap is 20 pf and I have had to go up to 100pf with the motorolas devices ElectroChap mentioned.

The hum can happen if the main ground screw to the power supply PCB/chassis is loose, or if someone has upgraded to new input connectors and used insulators for the jacks. The original design grounded the driver card through the connection of the jacks to the chassis. I see this problem fairly regularly.

Regards,
Mike

Thanks Mike,
I will leave the output transistors alone if everything measures fine.
The input conectors are the original ones, if I decide to replace them, I will not insulate.
Cant wait for the new caps to arrive to finish the job and to plug it to my Infinitys

Regards
Ricardo
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Old 7th December 2012, 12:41 PM   #13
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Originally Posted by Zero Cool View Post
I have one of these on my bench right now. I need to find schematics!
This schematics are the best ones I found... Not perfect, buit big enough to guess the components values.

RicardoClick the image to open in full size.
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Old 7th December 2012, 01:02 PM   #14
udailey is offline udailey  United States
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I bought one two weeks ago and haven't opened it yet, but upon getting it home I was disappointed in the bass, so will be working on it over the next few months. Glad to see others blazing a trail.
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Old 7th December 2012, 01:43 PM   #15
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Quote:
Originally Posted by udailey View Post
I bought one two weeks ago and haven't opened it yet, but upon getting it home I was disappointed in the bass, so will be working on it over the next few months. Glad to see others blazing a trail.
The supply flters and all the electrolytics are 35 years old. Also pull the 330uf bypassing the 150 ohm emitter resistors in the input stage... Also make sure the screw for the bridge rectifier is tight as this provides the chassis ground and reference for the input stages.

Bass is not a problem with this amp, unless you like it fat, slow

Regards,
Mike
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Old 7th December 2012, 01:48 PM   #16
udailey is offline udailey  United States
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I knew something was amiss when the bass was lacking. Thanks for the tips. What about using IEC plugs for this thing and using earth ground to chassis? Is this a problem?
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Old 7th December 2012, 02:13 PM   #17
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The original design didn't use a grounded chassis; the transformer design reflects this. I don't use the ground when I install an IEC plus on this model. I stick with the original design because when I have added the ground, it created grounding issues in the owners' systems that weren't there previously. Therefore, I'm faithful to the original design. If it's grounded, like the Ampzilla is, then I do use it.
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Old 19th December 2012, 08:23 PM   #18
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Hi all,
I finished the replacement of the caps, installed a nice power switch, moved the fuse inside the enclosure, calibrated the VU meters with a function generator and powered the SOA on... One channel is perfect, almost 0V DC on the outputs, the bias is a nice 30mV on all the resistors, everything fine. On the other channel, Ov DC on the outputs, but when placing the probes across any of the 4 resistors I have nothing, 0V! The supply is ok and I cant find any obvious fault in the circuit. Am I missing something obvious? Should I statart looking for new output transistors? The bias IC? This thing scares me a bit, since its impossible to find one...
Tomorrow I will test all transistors individually, but any ideas are very welcome.

Thanks
Ricardo
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Old 17th January 2013, 05:01 PM   #19
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My SOA is fixed and sounding great!
For those you need to replace the output transistors, please note that the GAS transistor cross reference has a small mistake.
The GAS118 is the NPN and the GAS119 is the PNP, use the schematics as a reference.
I ended up using MJ15022 and MJ15023 on the outputs with nice results.

Ricardo
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Old 18th January 2013, 05:35 AM   #20
SkyChu is offline SkyChu  Taiwan
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This's rev-0 schematic.
Son_Ampzilla_Rev0.gif
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